Self-igniting internal combustion engine

ABSTRACT

An internal combustion engine has compression ignition, in which fuel is injected as a pre-injection and main injection directly into a working space by means of an injection nozzle having a plurality of injection holes, with the pre-jection taking place in a clocked manner. In order to minimize the moistening of the walls of the combustion chamber, a combustion chamber is provided, in which are arranged an injection nozzle in the region of a cylinder center axis in the cylinder head and a piston recess arranged in the piston head, with an approximately centrally situated piston recess projection being arranged in the piston recess. The stroke of a nozzle needle of the injection nozzle is set in such a manner that a restricting of the range of the injected fuel jets is obtained.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for operating an internal combustionpiston engine, in particular a diesel internal combustion engine inwhich fuel is injected directly into a combustion chamber in a pluralityof fuel jets of a certain range by means of an injection nozzlecomprising a nozzle needle and injection holes, some of the fuel from aninjection cycle being injected in partial quantities during acompression cycle as a clocked pre-injection, and a remaining fuel beinginjected at a later time as a main injection into the combustion chamberat a higher injection pressure than during the pre-injection. Theinvention also relates to a combustion chamber design for carrying outthis method.

This application is related to application Ser. No. ______ (AttorneyDocket No.: 095309.55390US), filed on even date herewith and based onPCT/EP/03/02392.

In the modern internal combustion engines having self-ignition, the fuelis injected directly into a combustion chamber. In the case of acombustion of this type, there inevitably arise due to the heterogeneousmanner in which the combustion is conducted local zones in which thereis a virtually stoichiometric fuel/air mixture. High combustiontemperatures are produced in these zones, resulting in a high thermal NOformation. On the other hand, further zones which are rich in fuel andin which soot is formed arise. Given good turbulent mixing and an excessof air, some of the soot which is formed is re-oxidized, with completeburning off of the soot not being achieved.

German Patent Document DE 19953932 A1 (corresponding U.S. Pat. No.6,505,601) discloses a method in which a combinedhomogeneous/heterogeneous method of operation for obtaining average andrelatively high loads is proposed. In this method, the intention is forone injection strategy to be used both to permit an early homogeneousmixture formation in the compression stroke and also a subsequentheterogeneous mixture formation around the upper dead-center position,with the fuel injection in the case of the homogeneous mixture formationtaking place at a lower injection pressure than in the case of theheterogeneous mixture formation in order to avoid fuel from beingdeposited onto the cold walls of the combustion chamber.

It has nevertheless been shown that, despite the above-proposed measure,portions of the fuel pass onto the walls of the combustion chamber and,for the most part, do not participate in the homogeneous combustion, andlead to increased HC and CO emissions. Therefore, further measures haveto be taken to prevent any further moistening of the wall of thecombustion chamber with fuel.

The invention is based on the object of providing a method for aninternal combustion engine having self-ignition, with which a depositingof fuel on the walls of the combustion chamber is avoided. This isachieved according to the invention by providing a method of theabove-mentioned type, wherein the pre-injection is clocked in such amanner that, for each partial quantity, a range of the fuel jet in thecombustion chamber is restricted to be somewhat smaller than a distanceto a boundary of the combustion chamber, whereby a disintegration of theinjected fuel jets is reinforced at the same time, and wherein thecombustion chamber is bounded by a cylinder and a piston, said injectionnozzle being disposed in a region of a cylinder central axis and saidpiston including a piston head which in use faces the injection nozzleand includes a piston recess with an approximately centrally disposedrecess projection. Furthermore, it is the aim of the invention toprovide a combustion chamber design with which a self-igniting internalcombustion engine is improved in respect of the exhaust behavior and theconsumption. This is achieved according to the invention by providing acombustion chamber design which has an inwardly opening nozzle needleand a plurality of injection holes, wherein the combustion chamberincludes at least one outlet valve, and at least one inlet valve,wherein the stroke of the nozzle needle of the injection nozzle is setin such a manner that, within the injection nozzle, an effective flowcross section between the nozzle needle and the needle seat isapproximately 0.4 to 1.5 times an effective flow cross section of thesum of all of the injection holes.

Further refinements emerge from the subclaims.

According to the method according to the invention, the fuel is injecteddirectly into a combustion chamber by means of an injection device whichcomprises an injection nozzle having a plurality of injection holes anda nozzle needle. Some of the fuel of the particular cycle is present ina compression stroke of the internal combustion engine as a clockedpre-injection in a plurality of partial quantities in the form of fueljets having a certain range, the remaining fuel being injected as a maininjection at a later time. The fuel is optionally injected during themain injection into the combustion chamber at a higher pressure thanduring the pre-injection.

According to the invention, the pre-injection is clocked in such amanner that, for each injected partial quantity, the range of the fueljet in the combustion chamber is restricted. The range is somewhatsmaller than the distance to a boundary of the combustion chamber, witha disintegration of the injected fuel jets in the combustion chamberbeing reinforced. The individual injection cycles are configured duringthe pre-injection in such a manner that the jet pulses are matched ineach case to the individual injections, and, given the density of thegas in the combustion chamber at a particular instant, the range of thefuel jets is approximately the distance as far as the cylinder wall onthe combustion-chamber side, or the piston head. A depositing of fuel onthe wall is therefore avoided. The injection jet pulse and the partialinjection quantity are controlled by the duration of the pulse incombination with specific use of the throttling of the inflowing fuel inthe seat of the nozzle needle, with the result that the injected fueljets disintegrate because of reinforced atomization. As a result, thebest possible mixture homogenization of the injected partial quantitiestakes place, with a significant depositing of fuel onto the cylinderwall being avoided at the same time.

The higher pressure means that, for the heterogeneous portion ofcombustion, the criteria for an effective, conventional injection arefulfilled, since what is important here is a high jet pulse, intensivejet/wall interaction, and as good as possible utilization of air andturbulent mixing.

According to one embodiment of the invention, a stroke of the nozzleneedle of the injection nozzle varies during the clocked pre-injection.This permits a specific injection of fuel during the clockedpre-injection, as a result of which the operating range with purelyhomogeneous combustion can be further expanded. Furthermore, thevariation of the needle stroke permits a high degree of homogenization,since the accumulation of fuel particles on the walls of the combustionchamber is minimized. As a result, the knocking tendency can be reducedto a certain degree.

According to the invention, the pressure of the injected fuel can bevaried during the clocked pre-injection. In this case, the injectionpressure is preferably raised in order to counteract the rising pressurein the combustion chamber during the compression. This enables, forexample, the depth of penetration of the injection jets in thecombustion chamber to be kept constant during the clocked pre-injection.

According to a further variant of the invention, a cycle duration duringthe pre-injection is varied in such a manner that the partial quantitiesof fuel of the pre-injection differ. In this case, the variation can bedesigned in such a manner that the partial quantity of fuel injectedlater is larger than the previous partial quantity of fuel. Furthermore,according to the invention the last partial quantity of fuel of thepre-injection can be reduced in relation to the largest partial quantityof fuel that has previously occurred in the pre-injection in order tooppose a severe enrichment of the mixture cloud which has already beenhomogenized in advance.

For greater atomization of the fuel, according to the invention thestroke of the nozzle needle of the injection nozzle is varied, so thatan unstable, cavitating flow is produced in the injection holes of theinjection nozzle. As a result, an expansion of the injection jets andtherefore a better distribution of the fuel can be achieved.

For the specific setting of a desired throttling action in the seat ofthe nozzle needle and an unstable, cavitating flow, a suitablestructural measure, for example a double spring holder on the injectionvalve or a piezostrictive or magnetostrictive activation, can be used toassist in keeping the nozzle needle in a stroke position lying betweenthe completely closed and completely open position. In this case, theeffective flow cross section in the needle seat, i.e. between the nozzleneedle and the needle seat, should be approximately 0.4 to 1.5 times theeffective flow cross section of the sum of the injection holes.

By means of the mixture formation, obtained during the compressionstroke, of the pre-injected quantity of fuel, with a high excess of airduring the combustion, a significant thermal NO formation, and alsoformation of soot, are avoided, since the fuel is distributed finely andover a large area over the entire combustion chamber. For the maininjection, which is matched thereto and takes place at a later time, thethermal NO formation for the heterogeneous phase of combustion issignificantly reduced, because the concentration of oxygen is alreadysignificantly reduced by the preceding, homogeneous portion ofcombustion. An intensive, turbulent charging movement is preferablyinduced by the injection, this movement being assisted by the highinjection pressure.

An optimum homogenization of the pre-injected portions of fuel in thecompression stroke is achieved by the clocked pre-injection, with theresult that the fuel jets first of all injected in the combustionchamber evaporate, and then mix with air before the next jets follow.Since, with the increase in compression, the pressure in the combustionchamber likewise increases, more fuel is added during the followingclocking action.

In this case, the partial quantities occurring later in the form of fueljets are impeded by the increased pressure in the combustion chamberfrom passing onto the wall of the combustion chamber or boundary of thecombustion chamber. An increase in the injected quantity of fuel in thefollowing partial quantity is therefore made possible during theclocking action, which is brought about by means of an increase inpressure in the fuel injection pressure or by an extension of the needlestroke.

An expanded cycle duration also gives rise to an increase in thequantity of fuel inserted. The simultaneous combination of two or eventhree of the abovementioned measures would also be conceivable.

A decrease in the partial injection quantity during the last clockingaction may be advantageous in order to prevent a premature ignition ofthe homogenized mixture before the main injection occurs. Furthermore,the reduction of the last partial quantity during the clockedpre-injection can avoid over-enriching the mixture cloud which has beenhomogenized in advance.

In order to increase the rate of homogenization during thepre-injection, a swirling movement is produced in the combustionchamber, for example by means of a swirl inlet duct. It is the aim hereto offset or laterally displace or move a fuel cloud of an injected fueljet, which cloud is produced during an injection cycle, in such a mannerthat, during a following injection cycle, the newly injected fuel jetdoes not penetrate the fuel cloud of the preceding fuel jet.

According to one particularly advantageous embodiment, the pre-injectionfor the homogeneous portion of combustion takes place with clockingoccurring two to seven times in a combustion stroke range of approx.150° C.A to 30° C.A before the upper dead-center position. The number ofclocking actions and also the injection time of the first partialquantity can be varied as a function of the load.

By contrast, the main injection is carried out, for the heterogeneousportion of combustion, in a range around the upper dead-center positioneither as a block injection or with a different injection profile, withthe result that the flow of injected quantity of fuel is varied withinthe length of duration of the main injection in order to obtain a highpulse for the injection jets. A main injection with differing flow canbe obtained by means of pressure modulation and/or by varying the strokeof the nozzle needle. In order to satisfy the requirements for aneffective and heterogeneous combustion, the injection pressure ispreferably raised to a maximum level, for example between 1800 and 2400bar, for example by an injection device capable of pressure modulation.As appropriate, a short after-injection can follow directly after theclosing of the nozzle needle during the block injection in order toobtain a further reduction in soot. It is conceivable for both thepre-injection and also the main injection to take place with the samefuel pressure. For example, in a common rail system, a pressure level ofbetween 1000 and 1400 bar can prevail.

The after-injection is alternatively part of the main injection. Inorder- to obtain optimum combustion, the main injection and, ifappropriate, the after-injection take place successively as a functionof the load around the upper dead-center position in a range of 10° C.Abefore the upper dead-center position to 40° C.A after the upperdead-center position, with an opening duration of the nozzle needleduring the after-injection being set to be smaller than the openingduration of the needle of the main injection. It is also possibleoptionally for a late after-injection to take place which again does notparticipate in the combustion, and can serve only to regenerate anexhaust aftertreatment system connected downstream.

An injection strategy is proposed in the method according to theinvention making it possible to use an advantageous propagation of thefuel jet and mixture formation in a specific manner. Both homogeneousand heterogeneous combined combustion are obtained. In this case, amulti-hole-type nozzle is used. The injection pressure is preferablyadapted by means of a suitable injection system capable of pressuremodulation. In this case, a needle-stroke-controlled injection systemwith pressure modulation can be used.

Further criteria for designing an additional after-injection may arisefrom the requirements of a possible exhaust aftertreatment measure.

In order to carry out the method according to the invention, acombustion chamber is proposed which has, in the cylinder head, aninjection nozzle, which is arranged in the region of a cylinder centeraxis and has an inwardly opening nozzle needle, and a piston, with apiston recess having a centrally arranged compression projection beingsituated in the piston head.

According to the invention, a stroke of the nozzle needle of theinjection nozzle is set in such a manner that, within the injectionnozzle, an effective flow cross section between the nozzle needle andthe needle seat is approximately 0.4 to 1.5 times an effective flowcross section of the sum of all of the injection holes.

According to a preferred embodiment of the invention, the piston recesshas, from the piston head, first of all a flat inlet having a smallcurvature and, from the region of the maximum recess depth, a greatercurvature reaching into the piston recess projection. A transitionbetween the piston head and the piston recess is of rounded design. Thepiston recess projection has a conical shape with a rounded, bluntedpoint.

According to the invention, the moistening of the walls of thecombustion chamber during the clocked pre-injection is prevented in sucha manner that the injection nozzle arranged in the region of thecylinder center axis injects the fuel in the form of fuel jets with alimited range, and that the injected fuel does not moisten the pistonrecess arranged in the piston head to a significant extent.

The piston recess is of approximately plate-like design in its basicshape, with a projection extending from the center of the piston recessin the direction of the injection nozzle. The plate-like basic shapemeans that, in the piston recess, there are no narrow radii on thesurface or cross-sectional jumps in the piston material, with the resultthat, during operation of the internal combustion engine, if droplets offuel strike against the recess, they rapidly evaporate.

Since the recess depth comes less toward the outside diameter of thepiston and, at the transition to the piston head, a flat inlet with nosharp transistions of any type is provided, the accumulation of fuel isprevented.

The invention envisages that the shape of the combustion chamber and theconfiguration of the injection nozzles, in which a spray cone angle of90° to 160° can be set, make it possible, during the pre-injection whichtakes place at an early point, for an increased homogenization to takeplace and for a depositing of fuel on the cylinder wall to be greatlyreduced. At the same time, the geometrical shape of the combustionchamber has a very advantageous effect on the heterogeneous mixtureformation during the main injection, with the result that a combinedhomogeneous/heterogeneous operation can easily be brought about within aworking cycle using a conventional multi-hole-type nozzle.

According to the invention, the stroke of the nozzle needle can beadjusted in an opening direction, with the result that the stroke of thenozzle needle can be set in a variable manner during the clockedpre-injection. The stroke can alternatively be set as a function of theload. As a result, the injection quantity used during the clockedpre-injection is varied. Furthermore, the adjustment of the strokecauses an unstable, cavitating flow to be produced in the injectionholes of the injection nozzle.

At least one injection nozzle which is arranged approximately centrallyin the cylinder head of the internal combustion engine is proposed forthe injection of the fuel. This may in principle be a conventional andtherefore financially favorable hole-type nozzle of the seat-hole type,mini blind-hole type or blind-hole type.

According to one preferred embodiment of the invention, the injectionnozzle has six to fourteen injection holes which are distributed overthe circumference in one or two rows of holes. The distribution can takeplace uniformly. The injection holes are inclined in each case by anangle of 45° to 80° with respect to the nozzle axis. The spray coneangle is approx. 90° to 160°.

Further advantages emerge from the following description of the drawing.Exemplary embodiments of the invention are illustrated in the drawing.The description and the claims contain numerous features in combination.The expert will expediently also consider the features individually andcombine them into meaningful further combinations. In the drawing:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross section through an internal combustionpiston engine,

FIG. 2 shows a diagram of a fuel injection profile with pre-injectionclocked 5 times and increasing cycle duration and increasing needlestroke at a constant injection pressure, and also a main injection withafter-injection at an increased injection pressure,

FIG. 3 shows a diagram of a fuel injection profile with pre-injectionclocked 5 times and with the cycle duration remaining the same, at aconstant needle stroke and increasing injection pressure during thepre-injection, and also a main injection with after-injection at anincreased injection pressure,

FIG. 4 shows a diagram of a fuel injection profile with pre-injectionclocked 4 times and with increasing cycle duration at a constantinjection pressure, and also a main injection with after-injection at anincreased injection pressure,

FIG. 5 shows a schematic illustration of the effect of an unstable,cavitating flow in the nozzle hole of a multi-hole-type nozzle,

FIG. 6 shows a sectional illustration of the combustion chamber with thearrangement of the injection nozzle and of the piston recess during thepre-injection, and

FIG. 7 shows a sectional illustration of the combustion chamber with thearrangement of the injection nozzle and of the piston recess during themain injection.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an internal combustion piston engine 1, in which acrankshaft 2 is driven via a connecting rod 4 by a piston 5 guided in acylinder 9. A combustion chamber 8 which comprises a piston recess 6 letinto the piston head 7 is formed in the cylinder 9 between the piston 5and a cylinder head 10.

When a crank 3 of the crankshaft 2 is rotated in a crank circle 11 inthe clockwise direction, the combustion chamber 8 is reduced, with theair enclosed in it being compressed. The charge cycle in the combustionchamber 8 takes place via gas exchange valves and gas ducts (neitherillustrated) in the cylinder head 10.

With the crank 3 reaching an upper dead-center position 12, referred toas UDC below, the end of the compression is reached, in which thecombustion chamber 8 assumes its smallest volume and the maximumcompression pressure and the maximum compression temperature arereached. The current position of the piston 5 with respect to thecylinder head 10 is determined by the crank angle φ in relation to theupper dead-center position 12.

A multi-hole-type injection nozzle 13 is arranged centrally in thecylinder head 10. The injection nozzle 13 is activated by an electroniccontrol unit 16, the engine controller, via a signal line 15 and anactuator 14, for example a piezoactuator. The injection jets emergingfrom the injection nozzle are denoted by 17.

The fuel is made available by an injection pump 18 in a plurality ofpressure stages, a cut-off valve 20, expediently an electronicallyactivatable solenoid valve, restricting the respective maximum pressurein the fuel line 19.

A first embodiment of a fuel injection strategy for the internalcombustion piston engine 1 is illustrated in FIG. 2. The diagram shows afuel injection profile for a combined homogeneous/heterogeneousoperation with pre-injection PI clocked 5 times and increasing cycleduration at a constant injection pressure P₁ which is 500 bar, forexample. Furthermore, FIG. 2 shows a main injection MI and anafter-injection AI at an increased injection pressure P₂, with a maximumneedle stroke s being set during the main injection MI.

According to the injection strategy illustrated in FIG. 2, first of allat an injection pressure P₁ a clocked pre-injection PI takes place inthe compression stroke of the internal combustion piston engine 1 in acrank angle range of 80° C.A to approximately 35° C.A before UDC. Theclocked pre-injection PI takes place in such a manner that, during eachclocking action, a different needle stroke s is set. The specificclocking of the pre-injection PI results in a homogenization of theinjected partial quantities. An increasing setting of the needle strokeis preferred. The main injection and after-injection take place oneafter the other at a higher injection pressure P₂ in a region betweenUDC and approximately 30° C.A after UDC. During the main injection MI, ahigher needle stroke s is set than in the pre-injection PI, with theduration of opening of the needle during the after-injection AI beingset to be smaller than the duration of opening of the needle in the maininjection MI.

FIG. 3 is an illustration of a diagram in which an alternative injectionstrategy for the internal combustion piston engine 1 is shown. Itillustrates the fuel injection profile with which a combinedhomogeneous/heterogeneous operation is obtained with the pre-injectionPI clocked 5 times for homogenization with the cycle duration remainingthe same and with increasing injection pressure during the pre-injectionPI, and with a main injection MI with an increased injection pressure P₂with the needle stroke s set to the maximum and an after-injection AI.

The clocked pre-injection PI according to FIG. 3 takes place in thecompression stroke in a crank angle range of approximately 80° C.A toapproximately 35° C.A before UDC. It takes place in such a manner that,during each clocking action, the injection pressure increases, i.e.during the pre-injection PI a lower pressure prevails in a fuel line,for example in a common rail injection system, the line with the earlyinjection, than in the following injection, with the needle stroke sremaining constant during the clocked pre-injection PI. The maininjection and after-injection take place successively at a higherinjection pressure P₂ in a range between the upper dead-center positionand approximately 30° C.A after UDC. During the main injection MI, ahigher needle stroke s is set than during the pre-injection PI, with theduration of opening of the needle during the after-injection AI beingset to be smaller than the duration of opening of the needle in the maininjection MI.

One particularly advantageous injection strategy is provided by theinjection profile according to FIG. 4. A combinedhomogeneous/heterogeneous operation with pre-injection clocked 4 timesand with increasing cycle duration at a constant injection pressure isproposed therein, in which the nozzle needle 13 a remains in a lowerstroke position. Furthermore, a main injection MI at an increasedinjection pressure P₂ and a needle stroke s set to the maximum and alsoan after-injection AI is provided.

The clocked pre-injection PI takes place in the compression stroke in acrank angle range of 80° C.A to approximately 35° C.A before UDC. It isundertaken in such a manner that, during each clocking action, theinjection pressure P₁ remains constant. The needle stroke s similarlyremains constant during the clocked pre-injection PI. The main injectionand after-injection take place successively at a higher injectionpressure P₂ in a range between the upper dead-center position andapproximately 35° C.A after UDC. During the main injection MI, a higherneedle stroke s is set than during the pre-injection PI, with theduration of opening of the needle during the after-injection AI beingset to be smaller than the duration of opening of the needle in the maininjection MI.

The low injection pressure P₁ in the abovementioned injection strategiesaccording to FIGS. 2, 3 and 4 is selected in such a manner that theclocked pre-injection PI results in a homogeneous mixture formation, inwhich the injected fuel accumulates to an insignificant extent on theboundary of the combustion chamber 8.

In the above-described injection strategies, the main injection MI ofthe fuel in the region of the upper dead-center position serves for aheterogeneous mixture formation and permits an increase in the loadbeyond the load which can be obtained by the homogeneous portion. At thetime of the main injection MI, a cool-flame combustion of thehomogeneous portion is shut off and a hot-flame combustion takes place.The main injection is designed in such a manner that a temperature levelobtained by the main combustion does not lie in the region of increasedNOx formation (Zeldovich mechanism). The after-injection serves toreduce the soot particles produced, since a decrease in the maininjection quantity by the after-injection quantity makes it possible toprevent the formation of fuel-rich zones.

FIG. 5 is a schematic illustration of the injection nozzle 13 of theblind-hole-type nozzle type, where a nozzle of the seat-hole-type nozzletype would be just as suitable. In the injection nozzle 13 according toFIG. 5, the effect of an unstable, cavitating flow caused in a nozzlehole 21 of the injection nozzle 13 with a small needle stroke s of thenozzle needle 13 a, i.e. with the injection nozzle 13 partially open,and the resultant action on the angle of propagation of the injectionjet 17 are illustrated.

On the right-hand side in FIG. 5, the injection nozzle 13 is onlypartially open, as a result of which a throttling in the nozzle needleseat 22 is obtained. This throttling causes an unstable, cavitating flowin the nozzle hole 21 which leads to the angle of propagation α₁ of thefuel jet 17. In comparison to an injection nozzle with maximum strokesetting, as is illustrated on the left-hand side of FIG. 5, the angle ofpropagation α₁ due to the unstable, cavitating flow is greater than anangle of propagation α₂ which is brought about without such a flow. Theunstable, cavitating flow causes sharp fluctuations in the inside flow23 in the nozzle, these fluctuations leading, when fuel emerges from thenozzle hole 21, to a reinforced disintegration of the fuel jet andtherefore to a large angle of propagation α₁.

The fuel jet with the angle of propagation α₁ propagates in thecombustion chamber with intensive atomization, and therefore bringsabout better homogenization and a rapid evaporation of fuel, with theresult that more fuel can be injected in a partial quantity of thepre-injection PI without a significant moistening of the wall of thecombustion chamber.

By contrast, in the case of the injection nozzle 13 having the maximumstroke setting according to the left-hand side in FIG. 5, a stable,cavitating flow is formed. In the interior of the nozzle hole 21 on theleft-hand side, this flow causes a two-phase flow 24 which leads to anormal disintegration of the fuel. In comparison to a partially openinjection nozzle, the angle of propagation α₂ caused by the stable,cavitating flow is smaller than the angle of propagation α₁.

FIG. 6 shows the arrangement of the injection nozzle 13 and of a piston5 in the combustion chamber 8 during the clocked pre-injection. Thepiston 5 is situated in the internal combustion piston engine 1 in sucha position with respect to the injection nozzle 13 during thepre-injection PI that a fuel cone angle α in a range of 90° to 160°arises.

A piston recess 6 is let into the piston head 7. The piston recess 6 isof plate-like design, with a projection being situated in the center ofthe piston recess as a piston recess point 6 a. The piston recess point6 a is bordered by a recess base 6 b. The recess point 6 a projects inthe direction of the injection nozzle 13.

At the edge, the piston recess 6 has, as transition to the piston head7, a radius R1 which is preferably between 3 and 7 mm. The outer partsof the recess base 6 b are designed with a spherical radius R2 ofapproximately 45 mm. The transition from the piston recess point 6 a tothe piston recess base 6 b has a curvature with a radius R3 ofapproximately 20 mm.

The piston recess point 6 a is situated approximately opposite theinjection nozzle 13. The depth of the piston recess 6 increases from theedge of the piston recess 6 as far as the piston recess base 6 b. Thepiston recess point 6 a extends in relation to the injection nozzle 13in such a manner that it remains approximately below the piston head 7.The distance d1 between the upper point of the piston recess point 6 aand the piston head is approximately 6 mm, with it being possible ford2d1 to preferably be formed between 4 and 8 mm. The recess base depthd2 is approximately 18 mm. In the edge region of the piston recess 6,the plate-like basic shape of the recess has a rounded transition to thepiston head in order to avoid accumulations of fuel.

The piston recess point 6 a is of conical design with a recess coneangle β in the range of 90° to 130° and is designed with a rounded pointwith a radius R4 of approximately 5 mm. The fuel cone angle α and theposition of the piston recess point 6 a interact in such a manner thatthe propagation of the fuel jets is not disturbed by the piston recesspoint 6 a. This ensures that the fuel jets meet in the region of thepiston recess 6.

FIG. 7 illustrates the arrangement of the injection nozzle 13 and of thepiston 25 in the combustion chamber 8 during the main injection MI inwhich the piston recess point 6 a shows itself to be very advantageousfor the heterogeneous portion of combustion. The piston 5 is situated ina region about UDC in such a position that the injected fuel jets aredistributed within the piston recess 6 and are optimally ignited alongthe recess base 6 b.

The main injection MI of the fuel in the region of UDC serves for aheterogeneous mixture formation, a cool-flame combustion of thehomogeneous portion being shut off at the time of the main injection MIand a hot-flame combustion being initiated. The mixture formation of thepre-injected quantity of fuel, which formation is obtained in thecompression stroke, avoids, during the combustion with a high excess ofair, a significant thermal NO formation and the formation of soot, sincethe fuel is distributed finely and over a large area of the entirecombustion chamber. The shape of the piston recess has a veryadvantageous effect on the main injection MI, with the result that thethermal NO formation for the heterogeneous phase of combustion issignificantly reduced, since the concentration of oxygen is alreadysignificantly reduced by the preceding, homogeneous portion ofcombustion and an intensive, turbulent charging movement is furtherassisted by the piston recess.

The invention is based on an internal combustion engine havingcompression ignition, in which fuel is injected as a pre-injection andmain injection directly into a combustion chamber 8 by means of aninjection nozzle 13 having a plurality of injection holes, with thepre-injection PI taking place in a clocked manner. In order to minimizethe moistening of the walls of the combustion chamber, a combustionchamber 8 is proposed, in which are arranged an injection nozzle 13 inthe region of a cylinder center axis in the cylinder head 10 and apiston recess 6 arranged in the piston head 7, with an approximatelycentrally situated piston recess projection being arranged in the pistonrecess 6, and the stroke of a nozzle needle of the injection nozzle 13being set in such a manner that a restricting of the range of theinjected fuel jets is obtained.

1-14. (canceled)
 15. A method for operating a self-igniting internalcombustion piston engine, in which fuel is injected directly into acombustion chamber in a plurality of fuel jets of a certain range bymeans of an injection nozzle comprising a nozzle needle and injectionholes, some of the fuel from an injection cycle being injected inpartial quantities during a compression cycle as a clockedpre-injection, and remaining fuel being injected at a later time as amain injection into the combustion chamber at a higher injectionpressure than during the pre-injection, wherein the pre-injection isclocked in such a manner that, for each partial quantity, a range of thefuel jet in the combustion chamber is restricted to be somewhat smallerthan a distance to a boundary of the combustion chamber, whereby adisintegration of the injected fuel jets is reinforced at the same time,and wherein the combustion chamber is bounded by a cylinder and apiston, said injection nozzle being disposed in a region of a cylindercentral axis and said piston including a piston head which in use facesthe injection nozzle and includes a piston recess with an approximatelycentrally disposed recess projection.
 16. The method as claimed in claim15, wherein a stroke of the nozzle needle of the injection nozzle isvaried during the clocked pre-injection.
 17. The method as claimed inclaim 15, wherein the pressure of the injected fuel is raised during theclocked pre-injection.
 18. The method as claimed in claim 16, whereinthe pressure of the injected fuel is raised during the clockedpre-injection.
 19. The method as claimed in claim 15, wherein a cycleduration for the clocked pre-injection is varied for differentsequential clocked pre-injections, so that partial quantities of fuel ofthe pre-injection differ for the respective different clockedpre-injections.
 20. The method as claimed in claim 19, wherein the cycleduration of the different clocked pre-injections is varied so that apartial quantity of fuel injected later is larger than a previouspartial quantity of fuel.
 21. The method as claimed in claim 19, whereinthe last partial quantity of fuel of the last clocked pre-injection isreduced in relation to the largest partial quantity of fuel that haspreviously occurred in the pre-injections.
 22. The method as claimed inclaim 20, wherein the last partial quantity of fuel of the last clockedpre-injection is reduced in relation to the largest partial quantity offuel that has previously occurred in the pre-injections.
 23. The methodas claimed in claim 15, wherein a stroke of the nozzle needle of theinjection nozzle is varied in such a manner that an unstable, cavitatingflow is produced in the injection holes of the injection nozzle, as aresult of which an increased atomization of the fuel in the combustionchamber is obtained.
 24. The method as claimed in claim 19, wherein astroke of the nozzle needle of the injection nozzle is varied in such amanner that an unstable, cavitating flow is produced in the injectionholes of the injection nozzle, as a result of which an increasedatomization of the fuel in the combustion chamber is obtained.
 25. Themethod as claimed in claim 15, wherein a fuel cloud of a fuel jet, whichcloud is produced during an injection cycle, is offset or displacedlaterally during the pre-injection by means of a swirling movementformed in the combustion chamber, so that, during a following injectioncycle, the newly injected fuel jets do not penetrate the fuel cloud ofthe preceding injection cycle.
 26. The method as claimed in claim 23,wherein a fuel cloud of a fuel jet, which cloud is produced during aninjection cycle, is offset or displaced laterally during thepre-injection by means of a swirling movement formed in the combustionchamber, so that, during a following injection cycle, the newly injectedfuel jets do not penetrate the fuel cloud of the preceding injectioncycle.
 27. The method according to claim 15, wherein the piston recessis of a plate like design, with the projection extending from a centerof the piston recess in a direction toward the injection nozzle.
 28. Themethod according to claim 27, wherein a piston head surrounds the pistonend facing the injection nozzle, and wherein the piston recess includes:a flat inlet having a small curvature section starting at the pistonhead, and a greater curvature section extending from a maximum recessdepth to the projection.
 29. The method according to claim 28, wherein arounded design transition section is formed between the piston head andthe piston recess.
 30. The method according to claim 29, wherein theprojection has a conical shape with a rounded, blunted point.
 31. Themethod according to claim 15, wherein the projection has a cone angle ina range of 90° to 140°.
 32. The method according to claim 30, whereinthe projection has a cone angle in a range of 90° to 140°.
 33. Acombustion chamber for carrying out the method as claimed in claim 15,wherein the nozzle has an inwardly opening nozzle needle and a pluralityof injection holes, wherein the combustion chamber includes at least oneoutlet valve, and at least one inlet valve, and wherein the stroke ofthe nozzle needle of the injection nozzle is set in such a manner that,within the injection nozzle, an effective flow cross section between thenozzle needle and the needle seat is approximately 0.4 to 1.5 times aneffective flow cross section of the sum of all of the injection holes.34. The combustion chamber as claimed in claim 33, wherein the pistonrecess is of plate-like design, with the projection extending from thecenter of the piston recess in a direction of the opposite injectionnozzle.
 35. The combustion chamber as claimed in claim 34, wherein thepiston recess has, from the piston head, first of all a flat inlethaving a small curvature and, from the region of the maximum recessdepth, a greater curvature reaching into the piston recess projection.36. The combustion chamber as claimed in claim 35, wherein a transitionformed between the piston head and the piston recess is of roundeddesign.
 37. The combustion chamber as claimed in claim 36, wherein thepiston recess projection has a conical shape with a rounded, bluntedpoint.
 38. The combustion chamber as claimed in claim 37, wherein thepiston recess projection has a cone angle in a range of 90° to 140°. 39.A method of operating a self igniting internal combustion piston engine,comprising: injecting fuel directly into an engine combustion chamber ina plurality of fuel jets of a certain range utilizing an injectionnozzle having a nozzle needle and injection holes, said injectingincluding a plurality of sequential clocked pre-injections at respectivepre-injection pressures followed by a main injection at a higherinjection pressure than the pre-injection pressures, wherein the clockedpre-injections are controlled so that the injected fuel jets have arange smaller than a distance to a boundary of the combustion chamber,and wherein the combustion chamber is bounded by a cylinder and apiston, said injection nozzle being disposed in a region of a cylindercentral axis and said piston including a piston head which in use facesthe injection nozzle and includes a piston recess with an approximatelycentrally disposed recess projection.
 40. A method according to claim39, comprising: further reinforcing of disintegration of the injectedfuel jets during said pre-injections.
 41. A method according to claim40, wherein said further reinforcing of disintegration of the injectedfuel jets includes varying a stroke of the nozzle needle during theclocked pre-injections.
 42. Self-igniting internal combustion pistonengine apparatus, comprising: fuel injecting means for injecting fueldirectly into an engine combustion chamber in a plurality of fuel jetsof a certain range utilizing an injection nozzle having a nozzle needleand injection holes, said injecting including a plurality of sequentialclocked pre-injections at respective pre-injection pressures followed bya main injection at a higher injection pressure than the pre-injectionpressures, and control means for controlling the clocked pre-injectionjets to have a range smaller than a distance to a boundary of thecombustion chamber, and wherein the combustion chamber is bounded by acylinder and a piston, said injection nozzle being disposed in a regionof a cylinder central axis and said piston including a piston head whichin use faces the injection nozzle and includes a piston recess with anapproximately centrally disposed recess projection.
 43. Apparatusaccording to claim 42, wherein the projection has a cone angle in arange of 90° to 140°.
 44. Apparatus according to claim 42, wherein thepiston recess is of a plate like design, with the projection extendingfrom a center of the piston recess in a direction toward the injectionnozzle.
 45. Apparatus according to claim 44, wherein a piston headsurrounds the piston end facing the injection nozzle, and wherein thepiston recess includes: a flat inlet having a small curvature sectionstarting at the piston head, and a greater curvature section extendingfrom a maximum recess depth to the projection.
 46. Apparatus accordingto claim 45, wherein a rounded design transition section is formedbetween the piston head and the piston recess.
 47. Apparatus accordingto claim 46, wherein the projection has a conical shape with a rounded,blunted point.
 48. Apparatus according to claim 47, wherein theprojection has a cone angle in a range of 90° to 140°.